This invention relates to digitizer tablets, and in general to position determination using electromagnetic interactions.
Commonly assigned U.S. Pat. No. 4,368,352, whose contents are herein incorporated by reference, describes a conventional digitizer tablet of the travelling-wave type. In this technique, the parallel spaced conductors of a grid extending along one coordinate axis are sequentially pulsed while a coil in a pointing device--cursor or stylus--is positioned over the grid. The fields generated by the pulsed conductors induce signals in the coil, which signals are typically processed by synchronous rectification and low-pass filtering to provide a characteristic composite waveform whose amplitude increases to a maximum, reverses phase and then falls off. Coarse and fine positions of the coil center are determined by the time-dependent zero crossing of the composite waveform. The pulsing circuit typically employs a 50 percent duty cycle. That is, current is flowing to the grid conductors about one-half the time during each scanning cycle. The result is a tablet that has proven very satisfactory in terms of performance and cost but consumes a relatively large amount of power.
U.S. Pat. Nos. 3,732,369 and 4,713,496 describe digitizer tablets employing a different technology in which time-dependent signals are not used. In this technology, coarse location of a pointing device is determined based on the amplitude and phase of voltages induced in neighboring wires. Fine position is established by interpolation between the voltages associated with each of the spaced wires. There are some indications of a reduced duty cycle, but the amount of power consumed is still substantial.